Light-emitting device having a patterned substrate and the method thereof

ABSTRACT

This disclosure provides a light-emitting device including a patterned substrate and the manufacturing method thereof. The patterned substrate has a plurality of depressions and/or extrusions for scattering light emitted from a light-emitting layer. Each of the plurality of depressions and/or extrusions comprises a top portion, a bottom portion, and a sidewall portion enclosing the top portion and the bottom portion, and at least part of the sidewall portion comprises a curve. In a preferred embodiment, the light-emitting device further comprises a rough surface formed on at least one of the top portion, the bottom portion, and the sidewall portion.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 11/878,961, filed Jul. 30, 2007, which claims theright of priority based on Taiwan Application Serial Number 095127924,filed Jul. 28, 2006, the disclosure of which are incorporated herein byreference in their entireties.

TECHNICAL FIELD

This invention relates to a light-emitting device, and more particularlyto a light-emitting device having a patterned substrate and the methodthereof.

BACKGROUND OF THE DISCLOSURE

Light-emitting diodes (LEDs) have become a symbol of energy-savingindustry owing to their low power consumption and long operation life.Meanwhile, LEDs are generally expected to replace traditionalfluorescent illumination devices. Whether the LEDs can be broadlyimplemented in a variety of lighting apparatus or not depends on theirlight-emitting efficiency and cost. Therefore, how to increase the lightextraction efficiency becomes one of the decisive factors to improve thelight-emitting efficiency.

Surface roughening has been an effective means to enhance thelight-extraction efficiency, whether on a substrate or on a topsemiconductor layer. In the U.S. Pat. No. 6,091,085, as shown in FIG. 1,Lester discloses a light-emitting device having a surface-roughenedsubstrate. LED 10 comprises a sapphire substrate 11 and a plurality ofprotrusions 111 and depressions 112 formed therein for scattering lightemitted from a light-emitting layer 14. Therefore the light extractionefficiency of the light-emitting device is enhanced. The plurality ofprotrusions 111 and depressions 112 are formed by randomly rougheningthe surface of the substrate 11 by mechanically polishing orreactive-ion-etching (RIE). Although the light extraction efficiency isenhanced because of the roughened surface of the substrate 11, thesurface damages such as the alteration of the surface composition andthe lattice distortion are resulted from the roughening process as well.Besides, part of the mask materials or the reactive ions are sputteredonto and even doped into a certain depth of the substrate, therefore thequality of the epitaxy layer deposited thereafter is downgraded. Themechanical polishing process also damages the lattice structure of thesubstrate surface, such that the quality of the successively grownsemiconductor layer, such GaN, is worse, and the internal quantumefficiency is lowered.

Wang et al. published a paper in Journal of The Electrochemical Society,153(3) C182-185 studying a maskless method for Epitaxial LateralOvergrowth (ELOG) to grow GaN layer. The purpose of the paper is tosimplify the complexity of the traditional ELOG process using SiO2 as ahard mark. The study focuses on growing a GaN film having a latticestructure with lower defect density on a sapphire substrate, especiallywith lower threading dislocation defects (TDDs) caused by the latticemismatch between the sapphire substrate and the GaN film. Wet etchingscheme is utilized to form plural trenches of V shape or U shape, andfurther correlate the influence of the exposed facets of the sapphiresubstrate formed under different etching conditions upon the TDD densityof subsequently-formed ELOG GaN film.

SUMMARY OF THE DISCLOSURE

One aspect of the disclosure proposes a light-emitting device having apatterned substrate. The patterned substrate comprises an uneven surfacefor scattering light emitted from a light-emitting layer.

Another aspect of the disclosure proposes a light-emitting device havinga patterned substrate comprising a plurality of protrusions and/ordepressions. At least one of the depressions and/or extrusions comprisesa top portion, a bottom portion, and a sidewall portion enclosing thetop portion and the bottom portion, and at least part of the sidewallportion comprises a curve for facilitating the subsequent epitaxydeposition.

In a preferred embodiment, the surface of the depressions and/orprotrusions has fewer lattice damages. The sidewall portion comprises aplurality of symmetric planes. At least one of the top portion and thebottom portion comprises a flat plane.

In another preferred embodiment, the uneven surface further comprises arough surface formed on at least one of top portion, the bottom portion,and the sidewall portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a light-emitting device in accordance with the prior art.

FIGS. 2A to 2E demonstrate the steps of an exemplary method forfabricating the light-emitting device in accordance with the firstembodiment of the present disclosure.

FIG. 3 shows the cross-sectional view and the corresponding top view ofthe patterned substrate in accordance with the first embodiment of thepresent disclosure.

FIGS. 4A to 4D demonstrate the steps of another exemplary method forfabricating the light-emitting device in accordance with the secondembodiment of the present disclosure.

FIG. 5 shows the correlation curve of the output power dependent uponthe various etching time in accordance with the first embodiment of thepresent disclosure.

FIG. 6 shows the improvement of the output power of the presentdisclosure versus the conventional device in accordance with the firstembodiment of the present disclosure.

FIG. 7 shows the improvement of the external quantum efficiency of thepresent disclosure versus the conventional device in accordance with thefirst embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2A to 2E demonstrate an exemplary manufacturing method inaccordance with the first embodiment of the present disclosure. As shownin FIG. 2A, a substrate 21 is firstly provided wherein the substrate 21comprises at least one material selected from the group consisting ofsapphire, GaN, SiC, Si, GaAs, and other substrate material that can beused to grow III-V semiconductor on it. Subsequently, a patterned hardmask layer 22, such as silicon dioxide (SiO₂), is formed on thesubstrate 21. The patterned mask layer 22 can be achieved by forming aphotoresist layer on a hard mask layer pre-deposited on the substrate21, then performing lithography and etching process for defining andtransferring the predetermined pattern to the hard mask layer. Afterremoving the photoresist layer, the patterned hard mask layer 22 isformed as a protection film against the subsequent wet etching.

FIG. 2B shows the step for transferring the predetermined pattern fromthe patterned hard mask layer 22 to the substrate 21 to form a pluralityof depressions on the surface of the patterned substrate 21 a by a wetetching process. The wet etching process comprises the step of dippingthe substrate 21 having a patterned hard mask layer 22 thereon in aphosphoric acid (HNO₃) based solution. It is preferred to perform thewet etching process under high temperature condition such as 250 to 350°C. It is also preferred to mix a quantity of sulfuric acid (H₂SO₄) intothe HNO₃ based solution to increase the boiling point of the solution.It is also preferred that the proportion of HNO₃ in the solution shouldbe greater than H₂SO₄ so the HNO₃ can stay as a base. A preferred ratioof HNO₃ to H₂SO₄ is around 2:1. The proper operating temperature is setto around 250 to 350° C. Operating in lower temperature will result inlower etching rate and prolong the etching time; whereas, operating inhigher temperature will render the solution easily volatile, thereforealter the concentration of the etching solution, and affect thestability of the etching rate and product performance. Furthermore, thelife-cycle of the etching solution is shortened, therefore themanufacturing cost is increased. Moreover, high-temperature process is apotential risk to the public safety. According to one embodiment of thedisclosure, the etching rate is around 0.6 to 1 micron/min. under thecondition of operating temperature of 320° C. and the composition ratioof HNO₃ to H₂SO₄ being 2:1. The etching time is controlled within apredetermined time range to produce the depressions having a flat bottomportion, as shown in FIG. 2B.

After wet etching, the hard mask layer 22 is removed as shown in FIG.2C, and the plurality of depressions is formed on the patternedsubstrate 21 a. The top-view pattern of the depression comprises circleor ellipse.

Next, a semiconductor buffer layer 23 is epitaxially grown on thepatterned substrate 21 a as shown in FIG. 2D. The semiconductor bufferlayer 23 comprises at least one material selected from the groupconsisting of AlN, GaN, InGaN, AlGaN, and other III-nitride materials.The semiconductor buffer layer 23 is substantially filled thedepressions without forming cavities or voids to prevent the totalreflection at the interface between the semiconductor buffer layer 23and the voids. Therefore, the light is scattered by the depressions ofthe patterned substrate 21 a.

After the formation of the semiconductor buffer layer 23, alight-emitting stack layer 24 and a transparent conductive layer 25 aresequentially formed thereon, as shown in FIG. 2D. In the preferredembodiment, the light-emitting stack layer 24 is made of GaN basedmaterials and comprises a first semiconductor layer 241 of firstconductivity-type, an active layer 242, and a second semiconductor layer243 of second conductivity-type. The first conductivity-type is n-typeor p-type, whereas the second conductivity-type is different from thefirst conductivity-type. The structure of the active layer 242 can be adouble-heterojunction (DH) structure or a multi-quantum well (MQW)structure for achieving higher internal quantum efficiency. Thetransparent conductive layer 25 comprises metal or metallic oxide whichis selected from the group consisting of nickel, gold, indium tin oxide,cadmium tin oxide, antimony tin oxide, indium zinc oxide, zinc aluminumoxide, and zinc tin oxide.

A portion of light-emitting stack layer 24 is removed by theconventional lithography-etching process so as to expose a portion ofthe first semiconductor layer 241. A first electrode 27 is formed on theexposed first semiconductor layer, and a second electrode 28 is formedon the transparent conductive layer 25. In a preferred embodiment, themethod further comprises forming a reflecting layer 28 on the lowersurface of the patterned substrate 21 a for enhancing the lightextracted from the sidewall of the substrate. The reflecting layer 28comprises at least one material selected from the group consisting ofSn, Al, Au, Pt, Ag, Ti, Cu, PbSn, AuZn, SiO2, Al2O3, SiNx, and TiO2.

FIG. 3 shows the cross-sectional views of the depression of thepatterned substrate in different wet etching times. In one embodiment,the growth surface of the substrate comprises sapphire C plane. Theshape of the top pattern of the depression is circle. The weightpercentage of phosphoric acid to sulfuric acid is 2:1, and the operatingtemperature is about 320° C. The depressions 311, 312, and 313 areaccordingly formed with the etching time of 60, 90, 120 seconds. Thedepression 311 is composed of an opening top portion 311 a, a flatbottom portion 311 b substantially shaped as a triangle, and a sidewallportion 311 c comprising at least three symmetric planes. The sidewallportion 311 c encloses the flat bottom portion 311 b. The ratio of thebottom area of the flat bottom portion 311 b to the top area enclosed bythe opening top portion 311 a is about 0.35. The flat bottom portion 311b comprises sapphire C plane, i.e. {0001} plane, and the sidewallportion 311 b comprises sapphire R plane, i.e. {1-102} plane. Thecross-section of the sidewall portion 311 b near the top portion and thebottom portion is a curve for facilitating the subsequent epitaxydeposition, and the bottom corner of the depression is rounded. Thedepression 312 is similar to the depression 311 except for the ratio ofthe bottom area of the flat bottom portion 312 b to the top areaenclosed by the opening 312 a being about 0.18. The depression 313,different from the depression 311 and depression 312, is only composedof an opening portion 313 a and a sidewall portion 313 b. The sidewallportion 313 b comprises at least three symmetric planes intersected at abottom point O. The cross-section of the sidewall portion 311 b near thetop portion is a curve for facilitating the subsequent epitaxydeposition.

It is clear from FIG. 3 that the flat bottom portion is graduallydiminished and the sidewall portion is enlarged as the etching timeincreases. It is caused by the difference between the etching rate ofsapphire C plane and that of sapphire R plane. More specifically, theetching rate of sapphire C plane is higher than that of sapphire Rplane.

FIGS. 4A to 4D demonstrate another method for forming a patternedsubstrate in accordance with the second embodiment of the presentdisclosure. A substrate 41 is provided wherein the substrate 41comprises at least one material selected from the group consisting ofsapphire, GaN, SiC, Si, GaAs, and other substrate material that can beused to grown III-V semiconductor. Subsequently, a patterned photoresistlayer 42 is formed on the substrate 41 as shown in FIG. 4A. Thepatterned photoresist layer 42 is prebaked on a hot plate with atemperature around 160° C. for reflowing and reshaping the patternedphotoresist into a plurality of hemispheres, then performing etchingprocess such as dry etching for defining and transferring thepredetermined hemispherical pattern to the substrate as shown in FIG.4B. After removing the remaining photoresist layer, a patternedsubstrate 41 a is formed as shown in FIG. 4C. It is clear from FIG. 4Cthat the patterned substrate 41 a comprises an uneven surface composedof a plurality of extrusions. The top-view pattern of the extrusioncomprises circle or ellipse. Each of the plurality of extrusionscomprises a top portion, a bottom portion, and a sidewall portion. Thetop portion and the bottom portion are both flat and substantiallyparallel. The sidewall portion is a curve for facilitating thesubsequent epitaxy deposition. In a preferred embodiment, the topportion is further roughened by randomly etching the surface of the topportion to form an irregular pattern. The patterned substrate 41 b, asshown in FIG. 4D, includes additionally an irregular pattern on part ofthe uneven surface capable of further improving the light extractionefficiency. It is also preferred to form a regular pattern replacing theirregular pattern. Still, it is optional to form either the regular orirregular pattern on at least one of the bottom portion, the topportion, and the sidewall portion, or the entire uneven surface of thepatterned substrate in both the first and second embodiments.

FIG. 5 shows how the output power of the light-emitting device beenaffected by the patterned sapphire substrate formed under variousetching times in accordance with the first embodiment. It is clear fromthe figure that comparing with the flat substrate, the patternedsubstrates formed under the etching time of 30, 60, 90, 120 secondsrespectively have about 24%, 16%, 15%, and 5% improvement in the outputpower of the light-emitting device accordingly. The depression formedunder the etching time of 120 seconds has only 5% improvement due to thenear V-shape profile of such depression that does not facilitate thesubsequent deposition of the semiconductor buffer layer. Nevertheless,it is conquerable for one with ordinary skill in the art by tuning therelated parameters that are critical to the issue in the epitaxydeposition. It is preferred to form a depression having a flat bottomportion by controlling the etching time within a predetermined time. Forexample, the flat bottom portion of the depression can be formed underthe etching time of about 15 to 100 seconds, and the ratio of the bottomarea of the flat bottom portion to the top area enclosed by the openingof the depression is substantially between 0.15 and 0.75. Consequently,the output power is improved at least 10%.

FIG. 5 and FIG. 6 show another experimental results of the improvementof the external quantum efficiency and the output power by the effect ofthe patterned substrate according the present disclosure, which arecompared with the conventional flat substrate under various drivingcurrent conditions. It is clear from the figures that the light-emittingdevice in accordance with the present disclosure can achieve consistentenhancement no matter in external quantum efficiency or in output powerwhile comparing with the conventional light-emitting device with a flatsubstrate under the driving current of 0 to 200 mA.

It should be noted that the proposed various embodiments are not for thepurpose to limit the scope of the disclosure. Any possible modificationswithout departing from the spirit of the disclosure may be made andshould be covered by the disclosure.

1. A light-emitting device, comprising a patterned substrate having anuneven surface composed of a plurality of depressions and/or extrusions;a light-emitting stack overlaying the patterned substrate, comprising afirst conductivity type semiconductor layer, an active layer, and asecond conductivity type semiconductor layer, wherein at least one ofthe plurality of depressions and/or extrusions comprises a top portion,a bottom portion, and a sidewall portion, and at least part of thesidewall portion comprises a curve.
 2. The light-emitting deviceaccording to claim 1, wherein the sidewall portion comprises a pluralityof symmetric planes.
 3. The light-emitting device according to claim 1,wherein at least one of the top portion and the bottom portion comprisesa flat plane.
 4. The light-emitting device according to claim 1, whereinthe substrate comprises sapphire.
 5. The light-emitting device accordingto claim 4, wherein the bottom portion comprises a sapphire C plane. 6.The light-emitting device according to claim 4, wherein the sidewallportion comprises at least three symmetric sapphire R planes.
 7. Thelight-emitting device according to claim 1, wherein the top-view patternof the depression and/or extrusion comprises circle or ellipse.
 8. Thelight-emitting device according to claim 1, wherein the light-emittingstack comprises a III-nitride compound.
 9. The light-emitting deviceaccording to claim 1, wherein at least one of the top portion, thebottom portion, and the sidewall portion having a rough surface.
 10. Thelight-emitting device according to claim 9, wherein the rough surfacecomprises a regular pattern or an irregular pattern.
 11. Alight-emitting device, comprising a patterned substrate having an unevensurface composed of a plurality of depressions and/or extrusions; alight-emitting stack overlaying the patterned substrate, comprising afirst conductivity type semiconductor layer, an active layer, and asecond conductivity type semiconductor layer, wherein at least one ofthe plurality of depressions and/or extrusions comprises a top portion,a bottom portion, and a sidewall portion enclosing the top portion andthe bottom portion, and at least one of the top portion, the bottomportion, and the sidewall portion having a rough surface.
 12. Thelight-emitting device according to claim 11, wherein the rough surfacecomprises a regular pattern or an irregular pattern.
 13. Thelight-emitting device according to claim 11, wherein at least part ofthe sidewall portion comprises a curve
 14. The light-emitting deviceaccording to claim 11, wherein the sidewall portion comprises aplurality of symmetric planes.
 15. The light-emitting device accordingto claim 11, wherein the substrate comprises sapphire.
 16. Thelight-emitting device according to claim 15, wherein the bottom portioncomprises a sapphire C plane.
 17. The light-emitting device according toclaim 15, wherein the sidewall portion comprises at least threesymmetric sapphire R planes.
 18. The light-emitting device according toclaim 11, wherein the top-view pattern of the depression and/orextrusion comprises circle or ellipse.
 19. The light-emitting deviceaccording to claim 11, wherein the light-emitting stack comprises aIII-nitride compound.